Electrolytic production of fluorine



United States Patent 3,320,140 ELECTROLYTIC PRODUCTION OF FLUORINE Anthony W. Yodis, Whippany, N.J., assignor to Allied Chemical Corporation, New York, N. Y., a corporation of New York No Drawing. Filed Dec. 13, 1963, Ser. No. 330,230 4 Claims. (Cl. 20459) This invention relates to processes for making elemental fluorine.

A certain nitroso-hydrogen fluoride complex, i.e. NOF-3HF, a known compound, is a substantially waterwhite liquid having a boiling point of 95 C. Processes for making the same are disclosed in U.S.P. 3,062,902 of Nov. 6, 1962.

Commercial methods for making elemental fluorine on large scale involve electrolyzing HF of a KF-HF melt, electrolysis customarily requiring 7-9 volts. In accordance with the invention, it has been found that the NOF component of NOF-3HF is more easily electrolytically decomposed than is the HF constituent of the well-known KF-HF melt. NOF-3HF is a readily transportable, storable and handleable liquid. In view of the instant finding of the ease of electrolytic decomposition of the NOF constituent of NOF-SHF, the invention affords a notably advantageous starting material for making elemental fluorine. Moreover, it has been found that not only is the NOF constituent of the NOF-3HF readily amenable to electrolytic decomposition, but also that such decomposition may be effected by means of power requirements which may be half or less than those of the large-scale commercial operations utilizing the KF-HF melt as the source of fluorine. It has been found that the NOF constituent of the NOF-3HF electrolytically decomposed to produce elemental fluorine, at low voltages typically in the range of 3 to 4. Since elemental fluorine formation rates are a function of the amperage, and power requirements are a function of the product of amperage and voltage, the advantages afforded by the invention with regard to power requirements are self-evident.

In accordance with the invention, it has been found that electrolysis of NOF-3HF releases fluorine at the anode and nitric oxide (NO) at the cathode. Electrolysis reaction is believed to be represented by D'rect NOF-3HF NO (cathode)+%F (anode)+3HF Current The invention process may be carried out in any suitable cell preferably provided with separated anode gas and cathode gas compartments. Materials of cell body construction are those resistant to the action of NOF-3HF, and which may be various and include metals such as nickel, Monel, and magnesium, and synthetics such as polyethylene, polytetrafluoroethylene, and polychlorotrifluoroethylene. Of the metals, nickel is preferred, and of the synthetics, polytetrafluoroethylene is notably suitable. Anodic materials may be nickel or carbon, and cathodic materials may be nickel, Monel, or magnesium. The cell body may be made of cathodic metals, in which instance the cell body functions as the cathode which may be electrically associated with the negative side of a source of direct current. The cell lid or other convenient portions of the cell body may include suitable openings and pipe connections to facilitate charging of the cell with liquid electrolyte, flushing with inert gas, and discharge of product gases from the anode and cathode compartments to suitable separate anode gas and cathode gas collecting traps. Having regard for the hereindescribed process control features, design of specific cells is within the skill of the art.

Practice of the invention involves consideration of process control factors including temperature, electrolyte composition, current density, amperage and voltage.

Operating temperature should be such as to maintain electrolyte material in the cell in the liquid phase, keeping in mind particularly the C. boiling point of NOF-3HF. To prevent possible freezing, as a practical matter temperature in the cell should be not less than about 5 C. Desirably, operating temperatures are as low as possible consistent with economic cooling. It has been found that temperatures substantially in the range of 20-30 C. eifect smooth decomposition without consequential carry-over of HF with cell exit gases, and afford the notable advantage of operation without refrigeration.

As electrolysis proceeds, the NOF constituent of NOF-3HF decomposes as indicated, and the HF of NOF-3HF remains in the electrolyte liquor in the cell, i.e. as decomposition progresses, the electrolyte liquor tends to become richer in HF. Considerable HF accumulation in the cell liquor is tolerable. Undesirably high HF content in the cell liquor may be detected by appreciable carry-over of HF with the cell exit gases which condition in turn may be detected by substantial accumulation of HF in a cold trap in the fluorine exit line. However, to maintain best cell operation, it is preferred to continuously or intermittently draw off from the cell the electrolyte liquor, subject the same to distillation to distill off HF (B.P. 20 C.), and return the NOF-3HF, stripped of HP, to the cell.

Current densities may lie in the range of 10-100 lamps/sq. ft., more usually 10-80, and in many circumstances densities in the range of 10-40 are satisfactory. In order to reduce cooling requirements, high current densities are not referred. As current density increases, voltage increases, for example in a Well designed cell, voltage at about 10 amps/sq. ft. may be about 3.5, and at 40 amps/sq. ft. may be about 4 volts. Electrolyte decomposition does not ordinarily require more than 5 volts, which value applies to the electrolyte per se and does not include any additional voltage which might be needed in the case of a cell which, because of particular design, might offer unusually high resistance.

The gas in the fluorine exit line of the cell may be advantageously run thru a Dry Ice trap, the minus 78 C. temperature of which effects substantially complete condensation of any possible NOF-3HF and/ or HP carryover. The resulting gaseous exit of the cold trap is substantially pure fluorine.

The following illustrates practice of the invention.

The body of the cell employed was about 1 /2" x 6" x 12" deep, was made of nickel throughout, and Was electrically connected to function as cathode. The cell lid was provided with a depending skirt about 3" deep, arranged to project downwardly into the electrolyte and form, above the electrolyte level, an anode gas compartment, horizontal cross-sectional area of which was about x 5". The cell lid included suitable openings and pipe connections to facilitate charging of the cell with liquid electrolyte, flushing with inert gas, and discharge of product gases from the anode and cathode compartments. Compartment discharge pipes were arranged to pass anode and cathode product gases each thru separate NaF pellet scrubbers, and thence into separate gate collecting traps. The anode was a 4" x 10 X A nickel sheet projecting vertically downwardly from the lid, within the skirt, and into the electrolyte. The cell was charged with liquid NOF-BI-IF in amount such as to submerge the lower end of the skirt by about 1", electrolyte charge amounting to about 1400 cc. of the liquid nitrosyl fluoride-hydrogen fluoride complex. Cathode and anode compartments were each flushed with helium. During all of the following operations, no extraneous cooling was applied to either the cell or gas collecting traps. Direct current was put on the cell at about 5 amps. and 3.5 volts. Current density under these conditions was [about 11.4

amps./ sq. ft. After three hours, current, voltage and current density were changed to about 7.5 amps., 3.7 volts, and 17 amps/sq. ft. After a succeeding half hour, amperage, voltage and current density were altered to about amps., 3.8 volts and 22.8 amps/sq. ft. At the end of a succeeding half hour, current was shut off, the collecting traps were disconnected, and the cell was purged with helium. Up to this point, about 2324 ampere hours of current had been passed into the cell. After standing overnight, the cell was flushed with helium, gas recovery traps reconnected, and current turned on at about 7 amps. and 3.7 volts, with a current density of about 16 amps./ sq. ft. Operation was continued for about 3 hours, and for the subsequent 1% hours, samples of both anode and cathode gases were taken by allowing the gases to pass thru gas sampling bulbs. Infrared analysis of cathode gases showed NO plus only traces of N0 and N 0. The fluorine anode gases, after hydrolysis, were analyzed for nitrogen compounds by the Kjeldahl method and found to contain no nitrogen where sensitivity of the test method was about 0.1%. Electrolyte level in the cell was restored to about 1 submergence of the skirt by addition of fresh NOF-3HF. Power was turned on, and the cell was operated for about 3 hours at amperage of about 8-10, voltage of 3.7 to 3.8, and current density about 18.2 to 22.8 amps/sq. ft. Throughout the foregoing operations, temperature of the electrolyte was in the range of -24 C. The cathode compartment exit gas was substantially colorless (but became very brown in air), and was found to have a molecular weight of about 30.00 and 30.08 in two separate determinations by weighing a known volume of gas at known temperature and pressure, thus indicating substantially pure NO. Analysis of the anode compartment exit gas showed molecular weights of 37.48 and 37.55 in two separate determinations, demonstrating such gas to be substantially pure F I claim:

1. The process for making elemental fluorine which comprises introducing NOF- 3HF into an electrolytic cell, subjecting liquid-phase NOF-BHF in said cell to the action of a direct electric current to electrolytically decompose NOF-3HF and liberate elemental fluorine, and discharging elemental fluorine from said cell.

2. The process of claim 1 in which the liquid material in the cell is maintained at temperature substantially in the range of 530 C.

3. The process of claim 1 in which the liquid material in the cell is maintained at temperature substantially in the range of 2030 C.

4. The process of claim 1 in which the electrolytic decomposition reaction is carried out at a current density substantially in the range of 10-100 amperes/ sq. ft.

References Cited by the Examiner UNITED STATES PATENTS 2,693,445 11/ 1954 Howell et a1. 204- JOHN H. MACK, Primary Examiner.

H. S. WILLIAMS, Assistant Examiner. 

1. THE PROCESS FOR MAKING ELEMENTAL FLUORINE WHICH COMPRISES INTRODUCING NOF.3HF INTO AN ELECTROLYTIC CELL, SUBJECTING LIQUID-PHASE NOF.3HF IN SAID CELL TO THE ACTION OF A DIRECT ELECTRIC CURRENT TO ELECTROLYTICALLY DECOMPOSE NOF.3HF AND LIBERATE ELEMENTAL FLUORINE, AND DISCHARGING ELEMENTAL FLUORINE FROM SAID CELL. 